A BOLOMETER COMPRISING A MICRO-BRIDGE STRUCTURE AND A METHOD OF FABRICATING THE SAME

Abstract

A method of fabricating a micro-bridge device (14, 16) onto a substrate (20). The method includes the steps of: providing a sacrificial material (32) on a surface region of the substrate (20); patternwise etching the sacrificial material (32); providing a sensing material (34) on a surface region of the sacrificial material; providing a support material (36) on a surface region of the sensing material; and removing the sacrificial material (32) leaving support material (36) with the sensing material (34) on its lower surface, substantially free standing above the substrate (20).

Full Text

THE PATENTS ACT 1970
[39 OF 1970]
COMPLETE SPECIFICATION

[See Section 10; Rule 13]
"MICRO-BRIDGE STRUCTURE"-" _

QINETIQ LIMITED, a British company of 85 Buckingham Gate, London SW1 6TD, United Kingdom,
The following specification particularly describes the nature of the invention and the manner in which it is to be performed:-

Cr&.frr" 7^D

This invention relates to a method of making a micro-bridge and a new structure for a micro-bridge, such as may be used in imaging devices to detect incident radiation. The invention arose from the field of thermal imaging, but is not necessarily limited to that field.
Infra-red imaging cameras based on two-dimensional arrays of thermal detectors are attractive due to their near ambient temperature operation. Thermal detectors used for infra-red imaging rely on the temperature change of the sensing material due to absorption of infra-red radiation. A 1°C temperature change in the scene leads to a temperature change of about 0.001.C temperature change within the detector and it is therefore important to try and maximise the amount of radiation absorbed.
The sensing material has a temperature dependant property which allows the magnitude of the change in temperature to be detected, amplified and displayed using electronic circuitry. Examples are pyroelectric arrays, which rely on the change in electrical polarisation with temperature which occurs in ferroelectric materials, and resistance micro-bolometer arrays which utilise the change in electrical resistance with temperature which occurs in some materials
In all types of thermal detector it is advantageous to maximise the rise in temperature of the sensing material due to the absorption of infra-red radiation. The temperature rise is reduced by any thermal conduction mechanism which takes heat from the sensing material. This results in detector designs which maximise the thermal isolation of the sensing material. The requirements for electrical read-out and mechanical rigidity mean that, for most practical detectors, a physical connection is required

to the sensing material.
Infra-red (IR) imaging relies on the fact that all objects radiate energy with a peak wavelength depending on their temperature. For ambient temperature objects this peak wavelength is in the Infra-red at about l0um. Hotter objects radiate more intensely. IR imaging typically involves using lenses, which may be of Germanium, to collect and focus this radiation onto an array of sensitive elements placed in the focal plane of the optics. The elements are normally micro-capacitors or micro-resistors (micro-bolometers) whose characteristic parameter (charge or resistance respectively) depends on the temperature. The micro-bolometers are usually formed on silicon substrates using "micro-machining techniques". This involves depositing and lithographically patterning an active layer over a sacrificial layer that is finally etched away to leave a free-standing, thermally isolated structure.
Such a structure is shown in Figure 1 of the accompanying drawings wherein legs 2, 4 support the main body of the element 1 above a substrate (not shown). The legs 2, 4 ensure that mechanical support is provided for the main body but with low thermal conductivity to the substrate.
Each element generates an electric signal proportional to its temperature that, in turn, depends on theintensity of the IR energy absorbed by it or its adjacent layer. The electrical signals must then be read out using a circuit that will both filter and amplify.
Traditionally the- quantum devices used to detect IR radiation have required cooling to liquid nitrogen temperatures. The "uncqoled" technology described herein_operates_,at room temperatures. Since IR
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radiation is not obscured by smoke this technology is, also useful in fire-fighting applications. There are also applications where it is not necessary to form high-resolution images. The IR sensitive elements can be used for simple "intruder detectors" or fire detectors.
Presently there are two basic forms of the resistance micro-bridge. Firstly, there is the homogeneous bridge type, as shown in section in Figure 2a of the accompanying drawings. In this type of bolometer the bridge is formed from a material whose properties change as the temperature changes through radiation being absorbed. The change in material properties is determined in some manner, perhaps by measuring a change in current passing through the bridge. The skilled person will appreciate that whilst the homogeneous bridge can work satisfactorily well its performance may not be as efficient as desired.
The second class of micro-bridge can be termed "film-on-support" and provides a temperature dependent material (which may be resistive) as a film above a supporting bridge. Such a film on support micro-bridge is shown in section in Figure 2b of the accompanying drawings. The bridge absorbs incident thermal radiation of the wavelengths of interest causing a temperature change within the bridge which effects the resistance of the resistive material. Generally the resistive material will be a metal which when positioned above the bridge reflects some of the incident radiation reducing the sensitivity of the micro-bolometer. Further, the provision of the metal on top of the bridge requires vias to be fabricated through the bridge necessitating further processing steps.
An example of a micro-bridge structure is shown in the Journal of Microelectromechanical systems Vol.5 No. 4 December 1996 in an article by Shie, Chen, et al. However, the micro-bolometer shown therein is
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fabriicated by a process which is somewhat more complicated than may be desired. The bridge is formed over a V groove which is fabricated using an anisotropic wet etch.
A further micro bridge structure is also shown in US 5 698 852 Wherein a Titanium layer provides the resistor on the underside of a bridge formed from a layer of SiO2. However, this document shows the resistive bolometer portion sandwiched between two SiO2 layers. The micro-bolometer shown in this US patent has a much more complex structure than that described herein. It will be appreciated that simplifying the structure and processing steps reduces the cost of the device and also helps to increase the yield.
According to a first aspect of the invention there is provided a .method of fabricating a micro-bridge device onto a substrate including the step
a. providing a sacrificial material on a surface region of the
substrate;-
b. patternwise etching the sacrificial material;
c. providing a sensing material on a surface region of the sacrificial
material;
d. providing a Support material on a surface region of the sensing
material; and
e. removing the sacrificial material leaving the support material,
with the sensing material on its lower surface, substantially free
standing above the substrate.

This method is advantageous because it provides a micro-bridge structre by a method that has fewer processing steps than prior art methods. The skilled person will appreciate that the reduction of the number of processing steps is greatly advantageous because it will tend to increase
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the yield of the fabrication process and will also reduce the cost of devices fabricated by the method.
Advantageously, the support element is provided as a single layer of material and provides both physical support for the sensing material and acts as an absorber of incident radiation.
The sensing material may be a conductive material. Such a material is suitable for providing a micro-bridge structure in which a change of resistance is measured. Preferably the micro-bridge device is a micro-bolometer wherein the sensing material provides the resistor.
Alternatively, the sensing material may be a ferro-electric material. Such a material is suitable for providing a micro-bridge structure in which a change of charge is measured.
Preferably, the substrate has electronic circuitry provided therein. This is advantageous because it allows processing electronics to be provided for processing the signal from the micro-bridge and allows a single package to be provided containing both the micro-bridge and processing electronics. The provision of optimum processing electronics may not be possible in some prior art structures. For instance, in the paper by Shie, Chen, et al. the V~groove provided underneath the bridge structure is likely to prevent the provision of such electronics beneath the plan area of the bridge device - the region of the substrate where the electronics would be provided is etched away. Therefore, the structure provided by the method may provide an area beneath the micro-bridge device in which signal processing electronics may be located.
Most preferably the method is compatible with CMOS processing steps.

This is advantageous because it allows standard fabrication processes to be used which will generally reduce the cost of devices fabricated according to the method.
Conveniently step b of the process includes providing vias through the sacrificial material allowing connections to the electronic circuitry in the substrate. Such a step is a convenient way of providing the single package with processing electronics and micro-bridge. The skilled person will appreciate that the vias may be defined using a conventional photoresist to pattern the sacrificial layer or by the use of a photo-imageable polymer as the sacrificial layer.
In one embodiment the sacrifical material is polyimide which may be spin deposited and cred. The sacrificial material may be applied to a thickness of about 3um. However, in alternative embodiments the sacrificial material may be applied to a thickness of between about 1.5um and about 6um, or may be between about 2um and about 4.5um. It will be appreciated that the thickness of the sacrificial material governs the height of the sensing material above the substrate in the final micro-bridge structure.
The method may include the steps of using an etch solvent to ensure that all resist layers are removed subsequent to deposition of the sacrificial material. The etch solvent may be EKC.
The sensing material may be titanium (Ti). Titanium is advantageous because of its change of characteristics with temperature and also due to its low noise levels. Further, titanium has a high resistivity which may allow the design to be Optimised for CMOS circuitry more easily than other sensing materials. The skilled person will appreciate that for two
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equal portions of the material, the portion with the higher resistivity will have the higher resistance.
The sensing material may be deposited by sputter deposition, providing a i convenient method which is CMOS compatible. The sensing material may be deposited to a thickness of about 0.2um.
However, the skilled person will appreciate that other thicknesses may be suitable. For instance the sensing material may be provided to a thickness in the range of about 0.05u.m. to about 0.3um._.Or may be from about 0.1 um to about 0.25um. If the micro-bridge device is a micro-bolometer the sensing material may form the resistor. It is desirable that the resistance of this resistor is relatively high because this makes the signal provided from the micro-bolometer easier to process. Providing the sensing material in this range provides suitable resistances. Thicknesses greater than this range may tend to reduce the resistance too much.
The thicknesses and resistances described herein are particularly suitable when the sensing material is Titanium. If the sensing material is another material other thicknesses may be applicable.
In alternative embodiments the sensing material may be amorphous silicon, vanadium oxid platinum, nickel, aluminium or an alloy of any one of the aforementioned metals each of which provides suitable properties
The sensing material may have a sheet resistance of 3.3O/sq. Alternatively, the sensing material may have a sheet resistance of about 1.5£S/sq to about 6n/sq, or may be of about 2.5Q/sq to about 4.5Q/sq.

Conveniently step C. of the method includes the further step of removing the sensing material from optical alignment targets (OAT) provided on the substrate for alignment of subsequent layers. Such a step is advantageous because it simplifies the remaining steps of the method and makes it easier to position the remaining layers. It will be clear to the skilled person that OAT"s are necessary when a wafer stepper is used.
Step b of the method may also include the step of removing the sacrificial material from OATs provided on the substrate. Such a step is particularly advantageous should the sacrificial material be a material other than silicon dioxide.

Preferably the support material is deposited onto the surface region of the sensing material. This deposition process may be provided by Plasma Enhanced C^emicjilJ^pjHixr^^ Vapour Deposition (LPCVD), or by sputtering.
The support material may be deposited to a depth of about lu.m. This depth is convenient because it provides sufficient structural rigidity. However, the skilled person will appreciate that a range of other thicknesses may be suitable. For instance the support material may have a thickness of about 0.05um, O.lum, 0.5um to perhaps about 2um,3um,4|um,5um.
Preferably, the method applies the support material to an accuracy of about ± 10%. It will appreciated from the discussions above that the detector must be thermally isolated from its surroundings. Generally this is achieved by providing a bridge structure (of the support material) which is supported by a pair of legs. In such a structure the legs provide thermal contact between the bridge and the wafer or substrate. Having
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the legs too thick is disadvantageous because more heat is conducted from the wafer or substrate to the bridge which reduces the sensitivity of the micro bridge device. If the legs are ,00 thin then there is not enough mechanical support for the bridge structure. There is therefore a compromise between providing rigid supports and providing thermal
isolation.
The skilled person will appreciate that the micro bridge structure may be provided with a number of legs other than two. The micro bridge structure may be provided with 1,3,4,5,6,7 or more legs.
The method may comprise applying the support material to have a
thickness of about lA X where X is the wavelength of the incident radiation
of interest within the support material. The skilled person will appreciate
that the wavelength of the radiation will change according to the material
in which it is travelling. Applying the support material to this thickness
is advantageous because it causes destructive interference of radiation of
the wavelength of interest being reflected from the bottom surface of the
support material with radiation incident on the bridge. This destructive
interference promotes energy absorption and increases the temperature
rise of the support material due to the incident radiation of the
wavelengths of interest. . " .
The support material is conveniently patterned and etched to provide the necessary structures. Preferably, the resists used to pattern and etch the support material are removed by an etch solvent, which is conveniently EKC.
In one embodiment the support material is a silicon oxide which is
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advantageous because it is easy to provide using CMOS processing steps and readily absorbs radiation with a wavelength of about 8u.m to 14jj.ni which as discussed previously is the wavelength which is conveniently monitored by the micro-bridge device. It is advantageous that the support material absorbs the radiation to maximise the temperature change of the support material due to incident radiation of the desired wavelength. The skilled person will appreciate that for wavelengths other than 8um-14nm other materials which absorbs the wavelengths of interest may be advantageous.
The method may include the further step of providing a reflective layer on a surface region of the substrate. Such a layer may further enhance the efficiency with which the micro-bridge structure absorbs incident radiation.
The reflective layer may be provided on a surface region of the substrate before the sacrificial layer providing a convenient way of positioning the reflective layer. It will be appreciated that such a method provides a micro-bridge structure with the reflective layer provided on a top region of the substrate with the micro-bridge structure substantially suspended above the reflective layer.
Conveniently the method provides the reflective layer from a metal which may be any one of the following: Aluminium, titanium, nichrome, platinum, nickel or an alloy of any of these metals.
The reflective layer may be provided by sputtering, evaporation or any other suitable technique as will be appreciated by the person skilled in the art.
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Conveniently the sacrificial material is removed by ashing which may be performed in an oxygen plasma, providing an effective process for removing the sacrificial material without interfering with the layers covering the sacrificial material.
After step e. of the method a thermal anneal is preferably performed on the substrate. The thermal anneal is beneficial because it preserves the elevated value of the temperature coefficient of the resistance of the Ti, and may ensure that contacts within the circuits are formed correctly, etc. The thermal anneal may be provided by a rapid thermal annealing process or an industry standard furnace anneal.
The sensing material may be provided as at least one track. Preferably the method comprises providing the track such that incident radiation having a specific polarisation cannot pass therethrough. This may be achieved by arranging the track in a manner to block the passage of radiation having a polarisation. In particular, the track may be provided such that it has lengths running in directions transverse to one another. The track may be provided with substantial lengths running in directions parallel to one another, or may be transverse one another, or may be orthogonal one another. In other embodiments the method may comprise providing the track with curved portions. The curved portions may be circular, elliptical, etc.
A matching layer may be provided in a region above the support material, adapted to absorb incident radiation. The matching layer may be nickel chrome alloy and may be provided by evaporation, or may be sputtering.
A matching layer is advantageous because it may match the refractive index of the micro-bridge structure to that of free space. The skilled
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in CMOS processes and strongly absorbs electromagnetic waves at the wavelengths of interest.
The support element may be substantially square in plan, providing an efficient shape from which to provide an array of micro-bridge structures.
In one embodiment the support element is substantially square in plan and has dimensions of substantially 50u.m for the sides of the support element. In alternative embodiments the support element may have sides in the range of about 25um to about 100pm, or may be in the range 35u.m to about 75urn.
The support element may be provided with leg portions adapted to suspend the support element above the substrate. Such leg portions are advantageous because of the thermal isolation they provide for the support element from the substrate. Thermal isolation is advantageous because the temperature changes which must be measured by the micro-bridge structure are of such a magnitude that they are likely to be lost if the thermal mass of the substrate is not excluded.
Conveniently, the sensing material is also provided on the underside of the leg portions. Such a structure is again convenient because it provides a structure wherein the sensing material on the support element can easily be connected to circuit elements on the substrate.
Preferably the sensing material is a conductive material. In which case the sensing material may form a resistor.
Alternatively, the sensing material may be a ferro-electric material. In which case the sensing element may form the dielectric of a capacitor.
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Such structures are convenient because they provide means to measure temperature changes within the support element (i.e. a change in resistance of the resistor, or change of charge in the capacitor formed by the capacitor dielectric).
The resistor may be connected to a CMOS transistor provided in the substrate. This provides a convenient structure with which to process the information, provided by the micro-bridge structure.
The transistor may be arranged as a switch providing a convenient structure with which to connect the resistor to processing electronics at the correct instant for its resistance to be measured.
The resistor may have a resistance ofJ about 3kQ. Alternatively, the resistor may have a resistance in the range of about 1.5kCi to about 6kC2, or may be about 2kQ to about 4.5kQ.
The sensing material may be a metal and in particular may be titanium which is convenient material to provide using CMOS compatible processes. Further, titanium exhibits a temperature dependent resistance which makes it particularly suitable for this application and has a relatively high resistivity. In alternative embodiments materials such as amorphous silicon, vanadium oxide, platinum, nickel, aluminium, an alloy of any one of the aforementioned metals may provide the sensing material.
Preferably the resistor is provided as a track on the support element. This is advantageous because it allows the length of the resistor to be maximised which increases the value of the resistance which can be provided. Having a higher resistance is in itself advantageous because it